Our laboratory has demonstrated that exercise can significantly affect bone and muscle mass in elderly men and women. Regularly performed weight-bearing exercise can slow down or arrest the typical rate of bone loss seen in postmenopausal women. This may occur as a result of the stress of the exercise or by increased rates of calcium absorption. In addition, we have demonstrated that progressive resistance training an result in substantial increases in muscle strength and mass in a group of men aged 60- 72, as well as a group of men and women aged 87-96. We propose to examine the combined effects of a 9-month strength training program and increased vitamin D intake on total body Ca, N, K, C, and Na (measured by neutron activation in early postmenopausal women. The women will also be measured 12 months after completion of the exercise program and dietary inter- vention. Bone mineral content of the lumbar spine will be determined by dual photon absorptiometry and single energy quantitative computed tomography. The effects of the exercise and/or vitamin D on Ca absorption will be determined by measuring 47Ca absorption using the Tufts University whole body counter. In this study the relationship between muscle mass, bone content, and calcium absorption will be explored. We will also examine the effects of strength training ad dietary protein intake on the capacity of skeletal muscle of young and old subjects to hypertrophy. Two group of old (60-75) and young (18-30) subjects will reside in the metabolic research unit while they engage in strength training. One group will consume 0.8 grams of protein/kg per day while the second group will consume 1.5 grams of protein/kg per day. Dietary calorie intake will be similar for both groups and adjusted to maintain body weight. The residency and training period will be 12 weeks long. During this time, nitrogen balance will be determined. The total body nitrogen retention will be determined by measurement of total body N before and after residence. In this way, we will be able to accurately determine if positive N balance calculated from dietary intake - N excretion (urine, stool, sweat) is reflected in similar increases in total body nitrogen. These two studies will help determine the specific interaction between diet and exercise in preventing the typical age-related loss of bone mineral and muscle mass.